With recent advancement of digital technologies, electronic equipment such as portable information devices and home information appliances have been developed to provide higher functionality. For this reason, demands for an increase in a capacity of a nonvolatile memory element, reduction in a write electric power in the memory element, reduction in write/read time in the memory element, and higher retention property of the memory element are now increasing.
Under the circumstances in which there are such demands, it is said that there is a limitation on miniaturization of the existing flash memory using a floating gate. Accordingly, in recent years, an attention has been paid to a novel resistance variable nonvolatile memory element using a resistance variable layer as a material of a memory section.
FIG. 13 is a view showing a schematic configuration of a conventional resistance variable nonvolatile memory element. As shown in FIG. 13, a conventional nonvolatile memory element 600 has a very simple structure in which a resistance variable layer 602 is sandwiched between a lower electrode layer 601 and an upper electrode layer 603. Upon application of a predetermined electric pulse having a voltage with a magnitude of a certain threshold or larger, between the upper and lower electrodes, a resistance value between the upper and lower electrodes changes to a high-resistance value or to a low-resistance value (element performs a resistance changing operation). By corresponding these resistance values to numeric values (data), respectively, data is stored.
Because of such a simple structure and operation, further miniaturization and cost reduction of the resistance variable nonvolatile memory element are expected. Since changing between the high-resistance state and the low-resistance state occurs in a very short time of 100 ns or less in some cases, the resistance variable nonvolatile memory element has attracted an attention in terms of its high-speed operation, and a variety of proposals therefor have been heretofore made.
Recently, in particular, there have been numerous proposals made for resistance variable nonvolatile memory elements using metal oxides for the resistance variable layer 602. The resistance variable nonvolatile memory elements using such metal oxides are roughly classified into two major categories depending on the material used for the resistance variable layer.
One category is resistance variable nonvolatile memory elements disclosed in Patent Literature 1 or the like, which use perovskite materials (Pr(1-x)CaxMnO3 (PCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO), etc.), as the resistance variable layer.
The other category is resistance variable nonvolatile memory elements disclosed in Patent Literature 2 or the like, which use binary transition metal oxides (compound consisting of transition metal and oxygen). Since the binary transition metal oxides have a very simple composition as compared to aforesaid perovskite materials, composition control and layer deposition in manufacturing are relatively easy. In addition, the binary transition metal oxides have an advantage that they are relatively highly compatible with semiconductor manufacturing process steps. For these reasons, these days, the resistance variable nonvolatile memory elements using the binary transition metal oxides are under intensive study.
For example, Patent Literature 2 discloses resistance variable elements using as the resistance variable materials, oxides in stoichiometry of transition metals such as nickel (Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt (Co), iron (Fe), copper (Cu), and chromium (Cr), and oxides (hereinafter referred to as oxygen-deficient transition metal oxides) which are transition metal oxides and are less in oxygen content (atom ratio: ratio of the number of oxygen atoms to a total number of atoms, hereinafter the same occur) than the oxides in their stoichiometric compositions.
Patent Literature 3 discloses a resistance variable element using as a resistance variable material, oxygen-deficient tantalum (Ta) oxide. In this literature, it is reported that an element which performs a resistance changing operation is attained in a range of 0.8≦x≦1.9 (44.4% to 65.5% in terms of oxygen concentration) when an oxygen-deficient Ta oxide is expressed as TaOx.
Now, the oxygen-deficient oxides will be described in more detail. For example, in the case of Ta, Ta2O5 is known as an oxide in stoichiometry. Ta2O5 contains Ta atoms and O atoms in a ratio of 2:5 (atom ratio, hereinafter the same occurs). An oxygen content (atom ratio, hereinafter, the same occurs) of Ta2O5 is 71.4 atm %. Oxides having oxygen contents lower than 71.4 atm % are referred to as the oxygen-deficient oxides. The oxygen-deficient oxide in the present example is an oxide of Ta, and therefore may be referred to as an oxygen-deficient Ta oxide.
Patent Literature 4 discloses an example in which a resistance variable layer has a structure in which a surface region of titanium nitride is oxidized to form a titanium oxide (TiO2) crystalline layer of a nanometer order.
As stated above, a variety of materials are disclosed as the resistance variable layer materials. However, most of electrode materials sandwiching the resistance variable layer are precious metals, for example, gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). Non-Patent Literature 1 recites that the use of precious metals as the electrode material enable a resistance changing operation to occur more easily than the use of polysilicon or non-metal as the electrode material.
Among precious metals, in particular, Pt is used most frequently. All of Patent Literature 1 to Patent Literature 4 disclose resistance variable nonvolatile memory elements using Pt as electrodes. Pt may be regarded as one of most desirable materials as the electrode of the resistance variable nonvolatile memory element using a metal oxide layer.
In the examples disclosed in the cited prior arts, the thickness of the electrode layer comprising Pt is 100˜150 nm in Patent Literature 3 and 100 nm in Patent Literature 3.
Patent Literature 5 discloses a resistive memory element including a lower electrode, a resistive memory layer formed on the lower electrode, and an upper electrode formed on the resistive memory layer, and adapted to store a high-resistance state and a low-resistance state and switch between the high-resistance state and the low-resistance state by application of voltages, in which the lower electrode or the upper electrode includes a first conductive layer formed at the resistive memory layer side and comprising precious metal, and a second conductive layer which is in contact with the first conductive layer, is larger in thickness than the first conductive layer, and comprises a non-precious metal. According to Patent Literature 5, a thickness (layer thickness) of the electrode layer comprising precious metal is not less than 10 nm and not more than 20 nm.